<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            background-color: red;
            display: flex;
            overflow: auto;
        }

        .ab {
            background-color: green;
            width: 200px;
        }

        .ac::after {
            content: '';
            background-color: blue;
            width: 300px;
        }
    </style>
</head>

<body>
    <audio src="" controls></audio>
    <div class="a">
        <div class="ab">
            <div>ab</div>
            <div>ab</div>
            <div>ab</div>
            <div>ab</div>
            <div>ab</div>
        </div>
        <div class="ac">
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
            <div>ac</div>
        </div>
    </div>
</body>

</html>